1. Field of the Invention
The invention relates to a mechanical closure assist for a valve having a displaceable disc for engaging a valve seat to seal against fluid pressure. The closure assist can be mounted in or adjacent a valve casing carrying the disc and bears controllably against the disc for urging the disc against the seat.
2. Prior Art
Shutoff valves are known with a displaceable disc or similar valve body which may be operably engaged against a valve seat to seal against fluid pressure, or raised from the valve seat to allow flow between the disc and the structure defining the seat. These valves generally include a mechanical actuator for moving the disc into engagement with the seat.
In a simple construction for a shutoff valve, the disc is mounted on a rotatable shaft passing transversely across the valve, for example across a diameter of the disc. The disc can be rotated relative to the axis of the shaft or the like, either to position the edges of the disc against the valve seat and thereby occlude flow, or to pivot from the valve seat to allow flow along the disc, i.e., between the disc and the valve seat. The disc undergoes a simple pivoting movement through approximately 90.degree. from a fully opened position to a fully closed position, typically by rotating a shaft to which the disc is fixed. In the fully opened position the disc can be positioned such that its edge or minimum thickness intersects flow through the conduit or the like in which the valve is located. In the fully closed position the face of the disc occludes flow.
In a more complex arrangement for moving the disc in a shutoff valve, a linkage is included for moving the disc in both a pivoting movement for controlling whether or not the disc generally occludes the conduit, and through a linear displacement toward or away from the valve seat for sealing. Examples of such linkages are disclosed in U.S. Pat. No. 3,512,752--Uerlichs et al.; U.S. Pat. No. 4,586,693--Tinner; U.S. Pat. No. 4,634,094--Geiser; and, U.S. Pat. No. 4,770,392--Schmidt. Rotation on the shaft brings the valve disc into the vicinity of the valve seat, and the linkages provide linear movement of the valve disc for bearing more directly against the seat. The linear movement of the valve disc reduces friction between the disc and its seat, thereby reducing abrasion of these parts and providing longer disc and seat life. These linkages also urge the disc more tightly against the seat than is possible with a simple pivoting movement.
Pivoting valve disc mountings with displacement linkages suffer from the drawback that they are more complex than simple pivoting linkages, leading to additional expense and the potential for mechanical failure. Furthermore, these linkages place additional parts in the flow path through the conduit. Whereas with a simple rotating arrangement the cross section of the disc is minimal when the valve is open, the additional parts of a linkage with displacement features interfere with fluid flow through the open valve.
An additional problem arises when the shutoff valve is relatively large. As the diameter of the valve disc is increased, the torque applied to the central shaft must be increased to obtain the same amount of pressure between the sealing surfaces remote from the axis of rotation, as compared to a smaller diameter disc and the same amount of central shaft torque. Rotatable disc shutoff valves having a disc diameter of 48 inches or more are used in steam, air and other fluid flow systems. In a particularly demanding application, rotatable disc shutoff valves are used in systems for incineration of volatile organic compounds. The incineration process is carried out at such high temperatures that elastomeric seals are unsuitable for use in the valves. All sealing occurs at metal to metal abutting sealing surfaces within the valve casing.
It is known to incorporate a "step seat" in a valve casing for a pivotable or pivotable-and-displaceable valve disc. The step seat is an arcuate member or flange which extends radially inwardly from a wall of the structure defining the flow path through the valve casing. The step seat is structured to provide a sealing surface for a marginal outer portion of the valve disc. Normally, for a rotatable disc, a pair of step seats are provided to engage the valve disc on opposite axial sides relative to the direction of flow. The disc bears on the seat around the periphery of the valve casing, including at diametrically opposite sides of the valve casing remote from the disc rotation axis.
When the disc is rotated into the closed position, the disc contacts both of the step seats, but on opposite axial sides relative to the direction of flow. Therefore, one of the step seats is on a high pressure side of the disc, and the other of the step seats is on a low pressure side of the disc. Fluid pressure on the high pressure side of the disc helps to urge the disc against its step seat, toward the low pressure side of the disc and assisting in sealing. However, the fluid pressure on the high pressure side of the disk tends to detract from sealing on the opposite side of the disc. On the side where the step seat is on the high pressure side of the disc, the fluid pressure acts to lift the disc off the step seat. Only the stiffness of the disc resists fluid pressure from forcing one side of the disc to move away from its respective seat.
For a large diameter disc, even relatively modest pressure differentials can be sufficient to deform the disc so that it is lifted by a small amount from the step seat on the higher pressure side of the disc. In some applications, a gap of several thousandths of an inch can substantially detract from operation of the flow system including the valve. A disc of this type can be reinforced by adding material to stiffen the disc and resist deformation, but this solution results in a very heavy or perhaps unbalanced disc which is more difficult to rotate, requiring a more powerful actuator mechanism. The rotatable shaft and its bearings are subject to greater wear due to the increased loads caused by the heavier valve disc. Similarly, the disc might be formed such that the side tending to be lifted by pressure is preloaded by forming the disc so as to be nonplanar, i.e., such that the side tending to be lifted engages its step seat before the opposite side. The disc must then be shaped for a specific pressure, which is at best inconvenient, and is unworkable where the pressure is to vary.
There is a need for a device which will maintain a seal between the valve disc and valve seat without the drawbacks associated with a thickened, heavy valve disc or a complicated rotation/displacement mounting and other solutions for this problem in the art.
The invention provides a mechanical closure assist for a valve, operable for selectively contacting the valve disc at an outer edge to urge the valve disc into engagement with the valve seat. The mechanical closure assist according to the invention can be used with a disc of relatively light weight construction. The closure assist can be mounted in the valve casing adjacent the closed position of the disc, or in an adjoining section of conduit, so as to be fully retractable out of the valve flow path when the disc is in the open position, thereby minimizing resistance to flow through the valve. The closure assist acts on the edge of the valve which tends to be lifted by pressure, and provides a convenient and inexpensive means for overcoming the unequal effect of pressure on a stepped valve seat or similar arrangement.